In the practice of arc welding, it is extremely important that the welding current delivered to a weld electrode be substantially constant for a number of different reasons. Primarily, the current must be kept constant to prevent variations in the strength and integrity of the weld formed by the arc between the electrode and a workpiece. Unless the current is kept at a substantially constant value, the weld itself could be substantially weakened to the point that the weld could fail and cause serious damage to the structure or apparatus in which the weld is made and also cause personal injury where individuals are affected as the result of the failed weld.
The use of ramp voltages for controlling the current flow to a welding electrode of an arc welder has been known and practiced in the past. Ramp voltages generated for this purpose by conventional equipment are generally kept within a certain band of voltages; however, the ramp voltage, although held within such a band, drifts upwardly and downwardly within the band so that the net result is not a fixed, uniform voltage value but rather a generally continuously varying value. What this means in terms of electrode or welding current is that the current also drifts upwardly and downwardly. The reason for this drift is that, for a relatively small change of the ramp voltage, there is a corresponding increase or decrease in welding current which is relatively large, of the order of 2 to 5 amperes. Such a change in the welding current results in undesirable welds which are not uniform and give rise to structural defects of the type described above.
Conventional equipment for generating ramp voltages is complex in construction, expensive to produce and to maintain, and operates to cause drifts in the ramp voltage as described above. A typical ramp voltage generator of conventional construction includes an integrator which provides an upramp voltage and a downramp voltage and an amplifier for providing a ramp voltage of a substantially uniform value. However, switching from the integrator to the amplifier causes an error to be generated in the voltage, and this error requires frequent calibration of the output voltage of the generator, a time-consuming and expensive procedure. Thus, the ramp voltage generators of conventional construction do not provide a constant ramp voltage which can control the welding current in a manner to assure a weld of high quality and one which can withstand loads and other stresses without failure.
An off-the-shelf precision voltage source is not usable as a ramp voltage generator because it has no capability for generating upramp and downramp voltages. Such a voltage source would have to be extensively modified and the result would be a voltage source substantially the same as the conventional ramp voltage generator described above.
Because of the drawbacks of conventional ramp voltage generators and precision voltage sources, a need has arisen for improvements in techniques which provide for the generation of ramp voltages which can be used with an arc welder to assure a substantially uniform welding current to an electrode for generating a welding arc.